Abstract:

Systems and methods for sensing properties of a workpiece and embedding a
photonic sensor in metal are disclosed herein. In some embodiments,
systems for sensing properties of a workpiece include an optical input, a
photonic device, an optical detector, and a digital processing device.
The optical input provides an optical signal at an output of the optical
input. The photonic device is coupled to the workpiece and to the output
of the optical input. The photonic device generates an output signal in
response to the optical signal, wherein at least one of an intensity of
the output signal and a wavelength of the output signal depends on at
least one of thermal characteristics and mechanical characteristics of
the workpiece. The optical detector receives the output signal from the
photonic device and is configured to generate a corresponding electronic
signal. The digital processing device is coupled to the optical detector
and determines at least one of the thermal characteristics and mechanical
the characteristics of the workpiece based on the electronic signal.

Claims:

1. A system for sensing properties of a workpiece, the system
comprising:an optical input that provides an optical signal at an output
of the optical input;a photonic device coupled to the workpiece and to
the output of the optical input, the photonic device generating an output
signal in response to the optical signal, wherein at least one of an
intensity of the output signal and a wavelength of the output signal
depends on at least one of thermal characteristics and mechanical
characteristics of the workpiece;an optical detector receiving the output
signal from the photonic device and configured to generate a
corresponding electronic signal; anda digital processing device coupled
to the optical detector that determines at least one of the thermal
characteristics and the mechanical characteristics of the workpiece based
on the electronic signal.

2. The system of claim 1, wherein the digital processing device also
modifies a parameter in a process being applied to the workpiece in
response to the at least one of the thermal characteristics and the
mechanical characteristics of the workpiece determined by the digital
processing device.

3. The system of claim 1, wherein the photonic device comprises a
microring resonator.

4. The system of claim 1, wherein the photonic device comprises a defect
in a photonic crystal.

5. The system of claim 1, wherein the workpiece is one of a computer chip
and a mechanical product.

6. The system of claim 1, wherein the photonic device is embedded in a
layer of the workpiece.

7. The system of claim 1, wherein the photonic device has a Q between
approximately 100 and approximately 10.sup.5.

8. The system of claim 1, wherein the photonic device has a Q between
approximately 2,000 and approximately 20,000.

9. The system of claim 1, wherein the optical input comprises a laser.

10. The system of claim 1, wherein the optical detector comprises a
photodiode.

11. The system of claim 1, wherein the optical input is coupled to the
photonic device by a waveguide.

12. A method of embedding a photonic sensor in metal, the method
comprising:depositing a first optically insulating layer on a
substrate;depositing and patterning a waveguide material on the first
optically insulating layer to define the photonic sensor;depositing and
patterning a second optically insulating layer over the photonic
sensor;depositing a first metal layer over the second optically
insulating layer;etching the substrate to free the first and second
optically insulating layers, the photonic sensor, and the first metal
layer from the substrate and thus expose a surface of the first optically
insulating layer;depositing a second metal layer over the exposed surface
of the first optically insulating layer and thus substantially embed the
photonic sensor and first and second optically insulating layers between
the first and second metal layers.

13. The method of claim 12, wherein patterning the waveguide material
comprises at least one of lithography and etching.

14. A method for sensing properties of a workpiece with a photonic device,
the method comprising:providing an optical signal to the photonic
device;coupling the photonic device to the workpiece for which a
measurement is desired, the photonic device generating an output signal
in response to the optical signal, wherein at least one of an intensity
of the output signal and a wavelength of the output signal depends on at
least one of thermal characteristics and mechanical characteristics of
the workpiece; andreceiving the output signal from the photonic device,
and, based on the output signal, determining at least one of the thermal
characteristics and the mechanical characteristics of the workpiece.

15. The method of claim 14, further comprising modifying a parameter in a
process being applied to the workpiece in response to the at least one of
the thermal characteristics and the mechanical characteristics of the
workpiece determined.

16. The method of claim 14, wherein coupling the photonic device to the
workpiece comprises embedding the photonic device in a layer of the
workpiece.

17. A system for sensing properties of at least two regions of a workpiece
with at least two distributed photonic devices, the system comprising:an
optical input that outputs an optical signal;at least two distributed
photonic devices coupled to the least two regions of the workpiece and to
the optical input, the at least two distributed photonic devices
generating at least two output signals in response to the optical input,
wherein at least one of an intensity and a wavelength of each of the at
least two output signals depends on at least one of thermal
characteristics and mechanical characteristics of the at least two
regions of the workpiece;an optical detector receiving the at least two
output signals from the at least two distributed photonic devices and
configured to generate a corresponding electronic signal; anda digital
processing device coupled to the detector that determines at least one of
the thermal characteristics and the mechanical characteristics of the at
least two regions of the workpiece based on the electronic signal.

18. The system of claim 17, further comprising an optical waveguide that
couples the optical input to the at least two distributed photonic
devices.

19. The system of claim 17, further comprising an optical waveguide that
couples the at least two distributed photonic devices to the optical
detector.

20. The system of claim 17, wherein the at least two distributed photonic
devices are embedded in one or more layers of the workpiece.

21. The system of claim 17, wherein at least one of the photonic devices
comprises a microring resonator.

22. The system of claim 17, wherein at least one of the photonic devices
comprises a defect in a photonic crystal.

23. A system for sensing at properties of a workpiece, the system
comprising:means for outputting an optical signal;means, coupled to the
workpiece and to the optical signal, for generating an output signal in
response to the optical signal, wherein at least one of an intensity of
the output signal and a wavelength of the output signal depends on at
least one of thermal characteristics and mechanical characteristics of
the workpiece;means for detecting the output signal; andmeans for
determining at least one of the thermal characteristics and the
mechanical characteristics of the workpiece based on the detected output
signal.

24. The system of claim 23, wherein the means for generating an output
signal comprises one of a microring resonator and a defect in a photonic
crystal.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a U.S. national phase application under 35
U.S.C. §371 of International Patent Application No.
PCT/US2007/000133, filed Jan. 3, 2007 and entitled "Systems and Methods
for Sensing Properties of a Workpiece and Embedding a Photonic Sensor in
Metal," which claims priority to U.S. Provisional Patent Application No.
60/755,799, filed Jan. 3, 2006 and entitled "Distributed Subwavelength
Micro- and Nano-photonics for Ultrahigh Spatial- and Temporal-Resolution
in Displacement, Strain, Vibrational, and Thermal Sensing," the entire
contents of each of which are incorporated herein by reference.

TECHNOLOGICAL FIELD

[0003]The disclosed subject matter relates to systems and methods for
sensing properties of a workpiece and embedding a photonic sensor in
metal.

BACKGROUND

[0004]Placing temperature and strain sensors directly into a manufacturing
environment can help to obtain effective monitoring and control of
manufacturing processes for computer chip and mechanical products. If
critical conditions in these processes are continuously monitored with
sensors, problems can be detected and solved during the processing cycle,
resulting in improved product quality and productivity. There are ongoing
efforts to fabricate electrically based micro-sensors, such as thin film
thermocouples and strain gauges, for in-situ manufacturing process
monitoring. Typically, however, with electrically based sensor arrays
having a large number of sensors, e.g., that are distributed across a
computer chip or mechanical product, the assembly of wires that allow
readouts from these arrays can be cumbersome and costly. Monitoring the
fabrication of metal structures in hostile manufacturing environments is
particularly challenging because the presence of high temperatures,
corrosive agents such as acids, alkalis, and oxidizers, and
electromagnetic interference can damage an electrical sensor or impair
its ability to monitor processes.

SUMMARY

[0005]Systems and methods for sensing properties of a workpiece and
embedding a photonic sensor in metal are disclosed herein.

[0006]In some embodiments, systems for sensing properties of a workpiece
include an optical input, a photonic device, an optical detector, and a
digital processing device. The optical input provides an optical signal
at an output of the optical input. The photonic device is coupled to the
workpiece and to the output of the optical input. The photonic device
generates an output signal in response to the optical signal, wherein at
least one of an intensity of the output signal and a wavelength of the
output signal depends on at least one of thermal characteristics and
mechanical characteristics of the workpiece. The optical detector
receives the output signal from the photonic device and is configured to
generate a corresponding electronic signal. The digital processing device
is coupled to the optical detector and determines at least one of the
thermal characteristics and mechanical the characteristics of the
workpiece based on the electronic signal.

[0007]Some embodiments include one or more of the following features. The
digital processing device also modifies a parameter in a process being
applied to the workpiece in response to the at least one of the thermal
characteristics and the mechanical characteristics of the workpiece
determined by the digital processing device. The photonic device includes
a microring resonator. The photonic device includes a defect in a
photonic crystal. The workpiece is one of a computer chip and a
mechanical product. The photonic device is embedded in a layer of the
workpiece. The photonic device has a Q between approximately 100 and
approximately 105. The photonic device has a Q between approximately
approximately 2,000 and approximately 20,000. The optical input includes
a laser. The optical detector includes a photodiode. The optical input is
coupled to the photonic device by a waveguide.

[0008]In some embodiments, methods of embedding photonic sensors in a
metal include: depositing a first optically insulating layer on a
substrate; depositing and patterning a waveguide material on the first
optically insulating layer to define the photonic sensor; depositing and
patterning a second optically insulating layer over the photonic sensor;
depositing a first metal layer over the second optically insulating
layer; etching the substrate to free the first and second optically
insulating layers, the photonic sensor, and the first metal layer from
the substrate and thus expose a surface of the first optically insulating
layer; depositing a second metal layer over the exposed surface of the
first optically insulating layer and thus substantially embed the
photonic sensor and first and second optically insulating layers between
the first and second metal layers.

[0009]In some embodiments, patterning the waveguide material includes at
least one of lithography and etching.

[0010]In some embodiments, methods for sensing properties of a workpiece
with a photonic device include: providing an optical signal to the
photonic device; coupling the photonic device to a workpiece for which a
measurement is desired, the photonic device generating an output signal
in response to the optical signal, wherein at least one of an intensity
of the output signal and a wavelength of the output signal depends on at
least one of thermal characteristics and mechanical characteristics of
the workpiece; receiving the output signal from the photonic device, and
based on the output signal determining at least one of the thermal
characteristics and the mechanical characteristics of the workpiece.

[0011]Some embodiments include one or more of the following features.
Modifying a parameter in a process being applied to the workpiece in
response to the at least one of the thermal characteristics and the
mechanical characteristics of the workpiece determined. Coupling the
photonic device to the workpiece includes embedding the photonic device
in a layer of the workpiece.

[0012]In some embodiments, systems for sensing properties of at least two
regions of a workpiece with at least two distributed photonic devices
include an optical input, at least two distributed photonic devices, an
optical detector, and a digital processing device. The optical input
outputs an optical signal. The at least two distributed photonic devices
are coupled to the at least two regions of the workpiece and to the
optical input. The at least two distributed photonic devices generate at
least two output signals in response to the optical input, wherein at
least one of an intensity and a wavelength of each of the at least two
output signals depends on at least one of thermal characteristics and
mechanical characteristics of the at least two regions of the workpiece.
The optical detector receives the at least two output signals from the at
least two distributed photonic devices and is configured to generate a
corresponding electronic signal. The digital processing device is coupled
to the detector and determines at least one of the thermal
characteristics and the mechanical characteristics of the at least two
regions of the workpiece based on the electronic signal.

[0013]Some embodiments include one or more of the following features. An
optical waveguide that couples the optical input to the at least two
distributed photonic devices. An optical waveguide that couples the at
least two distributed photonic devices to the optical detector. The at
least two distributed photonic devices are embedded in one or more layers
of the workpiece. At least one of the photonic devices includes a
microring resonator. At least one of the photonic devices includes a
defect in a photonic crystal.

[0014]In some embodiments, systems for sensing properties of a workpiece
include: means for outputting an optical signal; means, coupled to the
workpiece and to the optical signal, for generating an output signal in
response to the optical signal, wherein at least one of an intensity of
the output signal and a wavelength of the output signal depends on at
least one of thermal characteristics and mechanical characteristics of
the workpiece; means for detecting the output signal; and means for
determining at least one of the thermal characteristics and the
mechanical characteristics of the workpiece based on the detected output
signal.

[0015]In some embodiments, the means for generating an output signal
includes one of a microring resonator and a defect in a photonic crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1(a) is a micrograph of a nanocavity according to some
embodiments of the disclosed subject matter.

[0017]FIG. 1(b) is a micrograph of a nanophotonic crystal sensor according
to some embodiments of the disclosed subject matter.

[0018]FIG. 2(a) is a schematic of an on-chip sensor system according to
some embodiments of the disclosed subject matter.

[0019]FIG. 2(b) shows micrographs of perspective and plan views of a
microring resonator according to some embodiments of the disclosed
subject matter.

[0020]FIG. 2(c) is a plot of the normalized transmission peak of the
microring resonator of FIG. 2(b) according to some embodiments of the
disclosed subject matter.

[0021]FIG. 3(a) shows micrographs of microring resonators according to
some embodiments of the disclosed subject matter.

[0022]FIG. 3(b) is a schematic of a microring resonator according to some
embodiments of the disclosed subject matter.

[0023]FIG. 4 is a schematic of a collection of information from photonic
sensor arrays according to some embodiments of the disclosed subject
matter.

[0024]FIG. 5 illustratively shows a method of embedding photonic sensors
in a metal according to some embodiments of the disclosed subject matter.

[0025]FIGS. 6(a) and 6(b) are micrographs of optical sensors embedded in
metals according to some embodiments of the disclosed subject matter.

[0026]FIG. 6(c) is a micrograph of a thermocouple embedded in metal
according to some embodiments of the disclosed subject matter.

[0027]FIG. 7 shows images of the transfer of a batch of photonic sensors
to a substrate and the embedding of the sensors in metal according to
some embodiments of the disclosed subject matter.

[0028]FIG. 8 is a schematic of a system for separating mechanically and
thermally induced strain components according to some embodiments of the
disclosed subject matter.

DETAILED DESCRIPTION

[0029]Systems and methods for sensing properties of a workpiece and
embedding a photonic sensor in metal are disclosed. In some embodiments,
photonic devices, such as microring and nanophotonic crystal resonators,
are useful for monitoring and controlling manufacturing processes for
pieces such as computer chips and mechanical products, by monitoring the
effects of thermo-mechanical phenomena arising from the manufacturing
processes on the pieces. More specifically, in some embodiments and as
described in greater detail below, the photonic sensors can be
incorporated into and distributed throughout the pieces during an early
manufacturing stage, e.g., before the pieces are exposed to processes
that would benefit from being monitored. The sensors can be made
sufficiently small, and can be selectively placed, such that they
generally will not interfere with normal operation of the finished piece.
In some embodiments, for example, the nanophotonic crystal-based
nanocavities described below, the sensors can be less than about 1 micron
in size, e.g., between about 100 nm and 1 micron in size, i.e.,
sub-micron, while in other embodiments, for example the microring
resonators described below, the sensors can be between about 1 micron and
about 10 microns in size, although other sizes are possible. Along with
the sensors, optical waveguides can also be incorporated into and
distributed throughout the pieces. These waveguides can allow an optical
input of known wavelength distribution, from an optical source such as a
laser, to be introduced to the sensors. The waveguides can also allow a
corresponding optical output that relates to the thermo-mechanical
effects of a process on the pieces to be collected from the sensors by a
detector. Based on the intensity and/or wavelength distribution of the
optical output, a computer processor in communication with the detector
can then appropriately modify the manufacturing process so as to adjust
the effects of the process on the pieces, for example, so as to inhibit
damage to the pieces or otherwise improve the process.

[0030]The photonic sensors can provide multiple features that are
difficult to obtain with conventional electrically based sensors. First,
because the photonic sensors have a significantly smaller physical mass
and thermal mass than conventional sensors, they can provide an enhanced
ability to sense process changes (e.g., strain, temperature, and
vibration). This speed of sensing allows those processes to be adjusted
as needed. Photonic sensors are also highly resistant to electromagnetic
interference and to hostile environments that can render typical
electrically based sensors inoperable. Photonic sensors also permit the
collection of data with spatial resolution and sensitivity that is
greatly enhanced over conventional macro-sensors, for example, allowing
spatial resolution on the order of 10 μm or better, strain resolution
on the order of 10-5, and temporal resolution on order of a
nanosecond or better. The output from multiple arrays of sensors can also
be multiplexed, as discussed in greater detail below. Photonic sensors
can also be embedded into metals to improve sensor survivability and
reliability in manufacturing environments. Metal embedded photonic
sensors can be fabricated with a batch fabrication process, and
subsequently transferred into larger metallic structures in manufacturing
environments. Minimally invasive physical protection (e.g., thermal,
photonic, and/or mechanical protection) can be added to the embedded
photonic sensors to improve their functionality. The sensors can further
be characterized after embedding, and calibrated under dynamic thermal
and mechanical loads.

[0031]One suitable kind of photonic sensor is based on resonators in
nanophotonic crystals. Nanophotonic crystals are periodic lattices of one
or more alternating dielectric materials that permit photonic band gaps.
These structures are analogous to the atomic potential lattices leading
to electronic band gaps in semiconductors. "Defects" in the periodicity
of nanophotonic crystals provide an exceptional means to design nanoscale
optical resonators from first principles. More specifically, an optical
resonator arising from these defects in periodicity resonates at an
optical frequency that is highly sensitive to the resonator's
thermo-mechanical environment. Thus, monitoring the resonant optical
frequency of the optical resonator provides information about that
environment that can be used to modify that environment appropriately,
e.g., by changing a process parameter.

[0032]FIG. 1(a) is a scanning electron micrograph of a nanocavity 120
within a nanophotonic crystal 100, in accordance with some embodiments.
The nanocavity can be used as a sensor for thermally and/or mechanically
induced strain. The dashed line around nanocavity 120 indicates the
defect region where the periodicity of the nanophotonic crystal is
disrupted. This defect region forms a nanoscale optical resonator that
has a characteristic operating wavelength λo. Light at an
integer multiple of this characteristic wavelength λo
resonates within the cavity, and experiences a relatively low loss. As
discussed in greater detail below, the presence of thermally and/or
mechanically induced strain modifies the characteristic operating
wavelength and the loss of the cavity, and thereby allows this strain to
be optically characterized.

[0033]The resonator can be characterized by its quality factor Q, which is
a measure of its extrinsic and intrinsic optical losses. The resonator Q
is defined as λ0/δλ, which is the ratio of the
resonator operating wavelength λo to the resonator full-width
half-maximum (FWHM) δλ. At the resonant wavelength, the Q of
a nanophotonic resonator is as high as 105, which is two to three
orders of magnitude higher than conventional fiber Bragg sensors, which
can also be used for temperature and strain sensing in manufacturing
environments.

[0034]Bragg sensors generally contain Bragg gratings as a sensing element.
Application of mechanical strain or a temperature change to these
gratings shifts the center wavelength of light reflected from the fiber
Bragg gratings, and the wavelength shift provides the necessary
information to calculate the change in strain or temperature. However,
Bragg sensors are typically significantly larger than photonic sensors,
e.g., between about 125 and 250 μm in size, versus less than about 10
μm in size for some embodiments of photonic sensors, and are
significantly less sensitive as discussed in greater detail herein.

[0035]The optical response time of a nanophotonic resonator, e.g., the
amount of time it takes a given amount of light resonating within a
nanocavity to decay below a threshold value, provides a measure of the
rate at which continuous strain event monitoring can be achieved.
However, a change in the temperature of the resonator, or the application
of a mechanical force, such as compression of the resonator, can cause a
strain that modifies the physical characteristics of the structure. This
can change the resonator Q as well as the resonant wavelength of the
cavity. Thus, a measurement of the resonator Q can be a measure of
localized strain.

[0036]In some embodiments, the Q of the resonator, and, thus, a
measurement of the thermal and/or mechanical strain experienced by the
resonator, can be determined interferometrically. For example, in some
embodiments, "signal" light of a first wavelength λ2 and
"control" light of a second wavelength λ1 are transmitted into
the crystal 100 via a waveguide 140 adjacent the nanocavity 120. The
first wavelength λ2 is selected to be sufficiently close to
the resonant wavelength of the nanocavity 120 that it couples into the
nanocavity 120. The light at the first wavelength λ2 interacts
with the cavity according to the Q of the cavity, and couples back into
the waveguide 140. The light at the second wavelength λ1 does
not significantly couple into the nanocavity 120, but instead simply
transmits along the waveguide 140. The light at the first and second
wavelengths λ2 and λ1 interfere with each other,
and the intensity of this interference is related to how much attenuation
the first wavelength λ2 experienced due to its interaction
with the nanocavity 120, and thus is related to the strain within the
nanocavity. This interferometric intensity is proportional to 1/Q. An
optical detector (not shown) in communication with the waveguide and
capable of detecting the change in interferometric intensity between
λ2 and λ1 arising from the strain within nanocavity
120 converts the optical interference signal into an electrical signal.
This electrical signal is transmitted to a computer processor (not shown)
that uses that information to calculate the strain within nanocavity.

[0037]When integrated onto a silicon chip, these nanophotonic crystal
resonators, like the microring resonators described below, allow the
thermo-mechanical strain experienced by the silicon chip to be monitored
with enhanced spatial and temporal resolution. The large resonator Q's
allow distinguishable Lorentzian resonance peaks under small strain or
thermal loading, permitting improved sensitivities.

[0038]Furthermore, due to the 1-dimensional geometry of these nanophotonic
crystal resonators, strain directionality can be obtained. For example,
FIG. 1(b) shows a nanophotonic crystal sensor 130 with sensing in both
the x and y directions (both .di-elect cons.x and .di-elect
cons.y) from the same nanocavity resonator 150 in accordance with
some embodiments. Sensor 130 includes waveguide 160 in the x-direction,
which transmits "signal" and "control" light at first and second
wavelengths, respectively, into nanocavity resonator 150 in the
x-direction. As discussed above for the sensor of FIG. 1(a), the "signal"
light at the first wavelength couples into nanocavity 150, and then
couples back into waveguide 160. The "signal" light interferes with the
"control" light at the second wavelength, and the intensity of this
interference is related to the strain experienced by the nanocavity 150
in the x-direction. A first detector (not shown) detects the
interferometric intensity, and a computer processor in electrical
communication with the detector analyzes the resulting signal. Sensor 130
also includes waveguide 170 in the y-direction, which transmits "signal"
and "control" light at third and fourth wavelengths, respectively, into
nanocavity resonator 150 in the y-direction. The "signal" light at the
third wavelength couples into nanocavity 150, and then couples back into
waveguide 170. The "signal" light interferes with the "control" light at
the fourth wavelength, and the intensity of this interference is related
to the strain experienced by the nanocavity in the y-direction. A second
detector (not shown) detects the interferometric intensity, and the
computer processor, which is also in electrical communication with the
second detector, analyses the resulting signal. The signals in the x- and
y-directions are effectively independent of each other, allowing the
separate measurements of the nanocavity's strain in the x- and
y-directions with minimal cross-talk.

[0039]Another suitable kind of photonic sensor is based on microring
resonators. Microring resonators are traveling wave resonators with a
transmission response that follows a Lorentzian line shape filter. FIG.
2(b) shows micrographs of a perspective view (top image) and plan view
(bottom image) of a fabricated microring resonator 210 in accordance with
some embodiments. Signal light from an optical source (not shown), e.g.,
a fiber optic source, travels along an input waveguide 230 that is
positioned adjacent to the microring resonator 210. The microring
resonator 210 becomes a resonant sink for the signal light when an
integral number of wavelengths match its optical circumference, i.e., at
a resonant frequency of the resonator. Light that couples from the input
waveguide into the resonator then couples from the microring resonator
210 into an output waveguide 220. This coupled light then travels along
the waveguide to an optical detector (not shown), e.g., a photodiode with
fiber optic input. The electronic detector output is optionally fed to a
lock-in amplifier to reduce noise, and is then input to a computer
processor for analysis. FIG. 2(c) is a plot 240 of the normalized
transmission peak of the microring resonator of FIG. 2(b), as monitored
at a detector in accordance with some embodiments. Plot 240 shows two
resonance frequency peaks of the microring resonator, and the distance
between the peaks corresponds to the free spectral range of the
resonator. As discussed in greater detail below, the central wavelength
and bandwidth of the transmission peak are related to the strain the
microring resonator experiences due to thermo-mechanical phenomena.

[0040]FIG. 2(a) is a schematic of an on-chip fabricated sensor system 200
in accordance with some embodiments. System 200 includes a microring
resonator 211, an input waveguide 231, an output waveguide 221, a fiber
optic detector 222, and a fiber optic source 232. Optionally, a secondary
detector 223, in optical communication with microring resonator 211 via
third waveguide 224, can be used for additional detection, e.g., for
monitoring the complementary transfer function output, as a back-up to
the detector fiber. This complementary transfer function output includes
signal light that does not couple into the microring resonator, the
intensity of which is reduced by the amount of light that does couple
into the microring resonator, thus providing a second method of measuring
the change in optical intensity due to optical coupling of signal light
into the resonator. V-grooves 260 can also be wet-etched anisotropically
on the chip, using standard CMOS processes, to facilitate reliable
alignment and placement of the fiber optic source 232 and detectors 222,
223 on the chip relative to the appropriate waveguides 231, 221, and 224,
respectively.

[0041]When a chip, having the embedded sensor system 200, is mechanically
or thermally compressed or stretched, changes in the output signals
recorded by the detector(s) can be tracked on GHz-frequency (ns
time-scale) commercial optical detectors, and monitored continuously and
simultaneously at numerous network grids of the manufacturing structure
through wavelength-division multiplexing. The on-chip sensors can also be
thermally loaded to finely map-out thermal variability and diffusion
responses on small lengthscales.

[0042]The change in the resonant frequency Δλ of the microring
resonator, with respect to applied strain .di-elect cons., is:

Δλ λ o = n eff [ 1 - 1 2 ( )
n eff 3 ] ( 1 ) ##EQU00001##

where λ0 is the resonant frequency, neff an effective
refractive index, and (.di-elect cons.) is the photoelastic coefficient
as a function of strain. For strains on the order of 0.1%, the
photoelastic correction term

1 2 ( ) n eff 3 ##EQU00002##

can be negligible, and the change in resonant frequency Δλ is
a linear function of strain .di-elect cons.. The ratio
(Δλ/λ0) is a linear function of temperature
variations, and related as:
Δλ/λ0=neff(α+β) ΔT/T, where
α is the linear thermal expansion coefficient and β the
photothermal coefficient expressing dependence of refractive index on
temperature.

[0043]The ring resonator can be separated from the waveguides by a gap
δ, which is designed and fabricated to be approximately equal to
the critical coupling distance for the resonator. A gap of 300 nm is
illustrated in the inset 310 of FIG. 3(a). The coupling coefficient
κ (the spatial overlap integral between the resonator and waveguide
modes) is exponentially dependent on the coupling gap δ, and at a
"critical" coupling distance, the coupling of light between the waveguide
and resonator is the largest. The value of the critical coupling distance
depends, among other things, upon the resonant frequency of the ring
resonator.

[0044]The presence of mechanical or thermal strain in the material changes
the size of the coupling gap δ, and therefore also changes the
value of the coupling coefficient κ. The resonator Q is related to
κ by

Q = 2 π 2 Rn e λ o κ 2 ,
##EQU00003##

where R is physical radius of the microring. Thus, a change in the gap
size δ (and therefore the value of K) leads to a large shift in the
resonator Q. This change in Q can be measured as a change in the light
recorded by the optical detector(s), and from that change the strain can
be determined. This provides a method for high-resolution strain
sensitivity in response to applied strain or thermal loading, with a
resolution proportional to 1/Q.

[0045]As compared to conventional fiber Bragg grating sensors, microring
resonators are significantly smaller in size (e.g., between about 1-10
μm for some embodiments of microring resonators versus between about
125-250 μm for Bragg grating sensors) and permit spatial resolution of
at least two orders of magnitude better. In addition, in some
embodiments, the quality factor Q of a microring resonator, typically
approximately 103 to 104, can be, e.g., a few times better, or
even ten times or more better, than conventional fiber Bragg grating
sensors, thus improving strain resolution. Microring resonators, like
nanophotonic crystal resonators, have temporal resolution on order 25-100
ps. Current detector technology permits measurement of modulation rates
up to 1011 Hz, and thus the temporal resolution measurable by the
system is limited mainly by the photon lifetime in the resonator. This
permits significantly improved temporal sensing and detection of
high-frequency small-amplitude strain variation, which is supported even
with commercial off-the-shelf photodetectors.

[0046]As compared to the 1-dimensional nanophotonic crystal resonators
shown in FIG. 1(a), microring resonators are inherently 2-dimensional
structures. Hence strain measurements from a single microring resonator
will yield the aggregate in-plane strain at its position. To achieve
strain directionality measurements, a differential structure 320 with
microrings in approximately orthogonal directions, as illustrated in FIG.
3(b), can be employed. Although in some embodiments microring resonators
sense with slightly lower spatial resolution and slightly lower strain
sensitivity than nanophotonic crystal resonators, the optical coupling
losses to microring resonators can be lower than for nanophotonic crystal
resonators, allowing for potentially easier characterization and
measurement. In some embodiments, ring resonators can have a Q between
about 2,000 and about 20,000. In some embodiments, nanophotonic crystal
resonators have a Q of about 70,000. In general, photonic resonators can
have a Q between about 100 and about 105.

[0047]Higher order resonator structures, such as structures 300 that shown
in FIG. 3(a), can be used to help to improve the sensitivity of the
sensor. Larger microrings, with larger radii, can be used to obtain
larger Q resonances for higher sensitivity.

[0048]In some embodiments, photonic sensors, e.g., nanophotonic crystal
and/or microring resonators, are integrated into a short closed-loop
optical waveguide (e.g., having a perimeter of approximately 50
mm˜100 mm) with an input-output coupling structure.

[0049]Note that using optical fiber input and output coupling to the
resonators provides data transmission at the speed of light between the
individual sensors and the data collection endpoints in the sensor
network. This provides distributed sensors with sufficiently high speeds
for demanding applications, for example, determining information about
critical locations of an aerospace structure, or about the high-speed
dynamics of fracture or ballistic impact. Since the photonic sensors are
fabricated on-chip, computational and decision logic transistors for
deployment of protective systems or corrective actions, for example, can
be integrated on-chip for minimal time-delay.

[0050]Photonic sensors can be defined through standard semiconductor
lithography techniques, permitting low-cost batch fabrication as well as
high precision location of the sensors on-chip. This provides for the
development of an on-chip parallel sensor array. As described above,
signal light can be introduced to the sensors via an optical fiber
attached to the chip, and a detector can collect light returning from the
sensors via a second optical fiber attached to the chip. As shown in FIG.
4, information from multiple arrays of the photonic sensors can be
collected over the same fiber bus through wavelength multiplexing.
Specifically, in some embodiments a single global light source can be
input to an array of microring resonators via an integrated waveguide
(curve below resonators), a part of which is adjacent the resonators in
the array. As the light traverses the waveguide, it couples to each
resonator in the array as it passes, providing an input to the resonator.
Similarly, as each resonator produces an output, that output couples to
an integrated waveguide (curve above resonators), a part of which is
adjacent the resonators in the array, and which feeds the output to a
single detector. The output relates to the responses of the different
resonators in the array. Commercial off-the-shelf photodetectors can have
nanosecond or better response times, supporting significantly improved
temporal sensing and detection of multiple high-frequency small-amplitude
signals from the resonators in the array.

[0051]The photonic resonators can be designed using a variety of analytic
and numerical techniques, including finite difference method (FDM) mode
solvers, finite difference time domain (FDTD) calculations, and
coupled-mode theory. With high index contrast waveguides, these
resonators can be made very small since radiation losses from tight bends
are greatly reduced. These photonic crystal and microring resonators can
be side-coupled or vertically coupled to the input and output waveguides
through evanescent fields. The resultant combination is a system with a
transfer function that has a single-pole response, i.e., that has a
single central resonance frequency, and narrow bandwidth. The location of
the pole, and hence the resonator Q and resonant frequency, is determined
by the input-output coupling ratios and the attenuation associated with
one optical transit phase within the resonator. These parameters are
optimized with analytic and numerical techniques prior to fabrication.

[0052]Photonic sensors can be fabricated on-chip with silicon-on-insulator
wafer substrates (hence permitting CMOS-level batch processing) and the
chips packaged to readily couple to signal (input) and detector
single-mode optical fibers. For example, microring resonators can be
fabricated in high index contrast silicon nitride and silicon material
systems using standard CMOS processes. In one method of making microring
resonators, a photoresist, e.g., PMMA, is coated over the silicon
material from which the sensor is to be made. Electron-beam lithography
is then used to define a pattern in the PMMA. The PMMA is developed and
subsequently metallized, with a liftoff step, to leave a patterned metal
mask over the silicon material. The assembly is etched through the entire
depth of the silicon material, and slightly into the insulator layer of
the substrate. Then the metal mask is removed using standard CMOS
processes to leave the microring resonator sitting on a "pedestal" of
insulator, which can help to optically isolate the sensor.

[0053]Recent advances in photonic device fabrication have made it possible
to construct very small microring resonators in a variety of materials,
including glass, polymers, silicon, silicon nitride (Si3NH4),
silicon oxynitride (SiON), and III-V semiconductors. The choice of
material depends upon the particular application. For example, in optical
signal processing applications like wavelength conversion and high speed
optical switching, materials with high nonlinear coefficients such as
AlGaAs/GaAs can be used.

[0054]In addition, note that in some embodiments, the photonic resonator
sensors, in the planar semiconductor fabrication process described here,
can be designed for operation in only one light polarization (either
transverse electric or transverse magnetic). Hence, the effect of
stochastic polarization due to mechanical strain on signal input-output
optical fibers might affect operation of the designed subwavelength
resonators and shift the resonance peak slightly. This polarization
dependence, however, can be largely removed using customized commercial
polarization maintaining (PM) fibers that adequately maintain the same
polarization over lengths of 1 kilometer or more.

[0055]In some embodiments, distributed microring and nanophotonic crystal
resonator sensors are selectively embedded at sensitive locations within
a piece, such as a computer chip or mechanical product, without
interfering with normal operation of the structure. This helps to avoid
directly exposing the sensor to external manufacturing environments, for
example, chemicals, moisture, and contamination. Many structures
fabricated in hostile manufacturing environments are metallic. Thus
embedding sensors in metal is one method of protecting them while using
them in a hostile environment.

[0056]One example of a procedure for fabricating and embedding sensors in
a metal in accordance with some embodiments is schematically depicted in
FIG. 5. First, at (1), Si3N4 is deposited using low-pressure
chemical vapor deposition (LPCVD) on a Si substrate. As shown in FIG. 5,
the Si3N4 is coated on both sides of the substrate. The
Si3N4 facilitates the release of the sensors from the substrate
later during the procedure. Plasma enhanced chemical vapor deposition
(PECVD) is then used to deposit a first layer SiO2, which acts as an
optical insulator for the sensor, and then a layer of SixNy,
e.g., the waveguide material, over the LPCVD Si3N4 film. In
general, SixNy, SiOxNy or another suitable material
can be used as the waveguide material, as discussed above and as known in
the art. In this example, the sensors are photonic devices made with the
PECVD SixNy film, and have an optically insulating structure
formed of SiO2. In general, any sensor, such as the photonic sensors
described herein can be embedded in metals using the described procedure.

[0057]Next, at (2), optical/electron-beam lithography and reactive ion
etching (RIE) are used to define the photonic sensors within the
waveguide material. In some embodiments, this is done by using
conventional lithographic patterning of photoresist (PR) as a mask
material, and RIE is subsequently performed to etch the waveguide
material in accordance with the pattern of the PR mask.

[0058]Next, at (3), the PR is stripped after the etching is complete and
SiO2 is then deposited using PECVD. This oxide layer, as well as the
one deposited at (1), are used to optically insulate the sensors from the
metal layers.

[0059]Next, at (4), The SiO2 is patterned using optical/electron-beam
lithography and RIE as described above, and as is known in the art. Ti
and Ni are optionally sputtered over the resulting structure as adhesion
layers.

[0060]Next, at (5), Ni is electroplated over the structure. The Ni layer
is relatively thick, e.g., about 1 μm, and is structurally robust. The
LPCVD Si3N4 at the bottom of the substrate is then dry etched
from the back side using RIE. Wet etching of the silicon substrate is
then done, followed by an RIE etch of the remaining Si3N4 layer
immediately beneath the sensors. This frees the sensors from the Si
substrate, effectively "transferring" them to the Ni, which then acts as
a new "substrate." Ti and Ni are then optionally sputtered as adhesion
layers over the newly exposed SiO2 surface, which previously was
attached to the substrate via the Si3N4 layer. Ni is then
electroplated over the patterned structure, substantially embedding the
sensors, plus the dielectric layers, within the metal. The sensors, thus
embedded, can then be readily coupled to a piece for which measurements
are desired, for example a computer chip or mechanical product. For
example, in some embodiments, a material for which measurements are
desired is grown on top of the sensor assembly.

[0061]Methods and materials other than those described can be used. For
example, electroplating can be selected to deposit metals for this
application because it works at near room temperature and generates
almost no stresses. However, other metal deposition processes can
additionally or alternatively be used, for example laser deposition.
Other metals, such as stainless steel, can also be used. In many
embodiments, the metal is compatible with standard CMOS process steps.

[0062]The number of sensors and their spatial distribution determine the
width and length of the embedding thin metal layers. Functionally
gradient thin film structures can be designed, selected, and fabricated
to optimize material properties such as thermal conductivity, thermal
stability, diffusion compatibility, strength, and thermal expansion,
between optical layers (e.g., SiO2) and metal layers (e.g.,
electroplated nickel). Appropriately selecting properties between
adjacent layers can, among other things, discourage delamination of the
layers and improve the performance of the device. With proper choice of
components and individual layer thicknesses, the property of a multilayer
coating can be tailored over the average of the two components. The
properties of different materials are well studied and documented.
However, the growth conditions of the materials (such as temperature,
pressure, and deposition rate) can also influence the properties of the
as-grown materials. As is known in the art, modest experimentation can be
performed in order to obtain "recipes" for growing materials that are
suitable for the particular application.

[0063]Optical sensors, such as SiO2-based Fiber Bragg Grating (FBG)
sensors, can be successfully embedded into metals, e.g. nickel and/or
stainless steel for temperature and strain measurements in manufacturing
processes, as shown in FIGS. 6(a) and 6(b). Single thin film
thermocouples (e.g., type K: alumel-chromel) can also be embedded into
metal, as shown in FIG. 6(c). The embedded sensor can be annealed at
800° C. for 3 hrs in argon. X-ray Photo-electron Spectroscopy
(XPS) before and after annealing can be used to show that the integrity
of the dielectric thin film Al2O3 and Si3N4 layers
are well preserved, indicating that photonic sensors fabricated from
these materials and embedded in metal will also survive. The sensor pads
on the thermocouples can be soldered with fine electrical wires, which
can then be connected to a PC-based data acquisition system for
calibration in a temperature-controlled oven.

[0064]Sub-micron optical thin films demonstrate improved properties over
their macro-optic counterparts. For example, amorphous submicron/nano
SiO2 and Si3N4 thin films have significantly higher strain
limit (3˜10%) than that of most metals (0.2˜1%). In some
embodiments, the core regions of the optical waveguides can be further
optically isolated from the metal surfaces in which they are embedded in
order to avoid unacceptably large optical losses due to optical
absorption by the metal. This isolation can be achieved by using a
cladding layer (for example, SiO2) of adequate thickness so that the
evanescent waveguide field will be sufficiently weak at the
cladding-metal interface. Standard analytic tools from integrated optics
can be used to compute the appropriate cladding thickness. Single-mode
waveguides cladded on both sides with a 2.0 μm silicon oxide show
negligible loss into the substrate.

[0065]FIG. 7 shows images of the transfer of a batch of photonic sensors
directly from a silicon wafer onto an electroplated nickel layer, and the
embedding of the sensors in metal. The process steps described above in
connection with FIG. 5 can be applied to a wafer having thin films
patterned across its surface using the batch fabrication of
metal-embedded photonic sensors in a clean room environment using
standard CMOS processes. Other kinds of structures, e.g., optical and
electronic devices, can also be embedded in metals using the described,
or similar, techniques.

[0066]As mentioned earlier, the photonic sensors described herein respond
to strain caused by both mechanical and thermal effects. FIG. 8 is a
schematic of a system for enhancing the sensitivity of microring photonic
sensors to mechanically induced strain by providing a thermal reference
scheme, in accordance with some embodiments. The system includes: a first
microring resonator 810; a second microring resonator 820, which is
suspended on a cantilever 830; a signal waveguide 840; and an output
waveguide 860. Signal waveguide 840 is split by a "y-splitter" into first
and second signal waveguides 841 and 842, which lead to first and second
microring resonators 810 and 820, respectively, substantially as
discussed above relative to the single microring resonators. Output
waveguide 860 is split by a "y-splitter" into first and second output
waveguides 851 and 852, which lead from first and second microring
resonators 810 and 820, respectively, substantially as discussed above
relative to the single microring resonators. The system also includes an
optical source (not shown) that is coupled to signal waveguide 840 (and
thus to first and second signal waveguides 841 and 842), and a detector
(not shown) that is coupled to output waveguide 860 (and thus to first
and second output waveguides 851 and 852).

[0067]First microring resonator 810 experiences both mechanical and
thermal strain effects arising from its environment. However, second
microring resonator 820 is suspended on cantilever 830, and thus does not
experience significant mechanical strain due to, e.g., compressive forces
Fcompression on the system, although it remains thermally coupled to
the system and thus experiences thermal strain effects. In some
embodiments, signal light from an approximately single-wavelength optical
source is split at the "y-splitter." A portion of the signal light is
input to the first microring resonator 810 via first signal waveguide
841, and another portion of the light is input to the second microring
resonator 820 via second signal waveguide 842. Because the first and
second microring resonators 810 and 820 experience different strain
effects, they couple differently to the signal light, e.g., with
different coupling efficiencies. The light that couples to each of the
microring resonators then couples as output into the corresponding first
or second output waveguide 851 or 852. Where the first and second output
waveguides join at the "y-splitter," the respective outputs from the
first and second microring resonators 810 and 820 interfere with each
other, when the optical path lengths (841+810+852) and (842+820+851) are
designed and fabricated to be sufficiently similar to permit interference
between the output signals. The thermal signal 862 from the second
resonator 820 destructively interferes with the thermal component of the
overall (thermal plus mechanical) signal 861 from the first resonator
810, such that the main remaining signal component is the mechanical
signal as measured by the first resonator. Some embodiments include a
subsystem for adjusting the relative optical phases of the output signals
from the two resonators in order to enhance their destructive
interference, optionally under control of the computer processor. Note
that in the illustrated embodiment, the direction of strain is orthogonal
to the direction of light propagating through the system, although other
configurations can be used.

[0068]In other embodiments, the first and second microring resonators 810
and 820 are designed and fabricated so as to have slightly different
resonant wavelengths, and the signal light includes first and second
wavelengths that are at or near the resonant wavelengths of the
respective resonators. After the signal light couples to the resonators,
the resulting output signals are joined together by the "y-splitter" and
sent to the detector, where the thermal signal as measured by the second
microring resonator 820 are normalized away, or otherwise subtracted
from, the overall signal (thermal plus mechanical) as measured by the
first microring resonator 810.

[0069]In general, different embodiments of photonic sensors, e.g., the
nanophotonic crystal and microring resonators described herein, or other
types of sensors, can be implemented in this scheme, and other
interferometric or other kinds of input/output arrangements are possible.

[0070]In summary, photonic sensors, such as nanophotonic crystal and
microring resonators, can be individually designed for a particular
application, and can have strain sensitivities on the order of 10-5.
The strain sensitivity is inversely proportional to Q, and hence the
sensors are designed to have as large a Q as possible. In some
embodiments, photonic resonators are designed to operate in an
interference scheme with two coupled resonators, and, thus, have an even
higher strain sensitivity, e.g., up to order 10-8 strain. The
photonic sensors can be fabricated using standard CMOS processing, and
can be embedded in metals to optimize their performance in hostile
manufacturing environments. The photonic sensors can be used to measure
in-situ strain and temperature effects on computer chips and mechanical
elements as they are being fabricated, and can also be used to measure
residual strains in the chips and elements after fabrication is
completed. Although nanophotonic crystals and microring resonators of a
particular design are described herein, other structures that serve as
photonic sensors, and that can optionally be embedded in metals, can also
be used in some embodiments.

[0071]In view of the wide variety of embodiments to which the principles
of the present invention can be applied, it should be understood that the
illustrated embodiments are illustrative only, and should not be taken as
limiting the scope of the present invention. Features of the present
invention can be used in any suitable combinations.